1. Field of the Invention
The present invention relates to a bias voltage modulation system such as a FSK modulator, etc. (specifically, FSK modulator, QPSK modulator, SSB modulator, etc.) and the like. In particular, the present invention relates to a modulation system and a modulation method which enable to obtain an optimal operating condition automatically by effectively adjusting bias voltage by using an optical modulator having Mach-Zehnder (referred to below as MZ) waveguide and the like.
2. Description of the Related Art
In optical communication, light must be modulated to be converted to signals. As optical modulation, direct modulation and external modulation are known. The direct modulation modulates a driving power of semiconductor laser. And the external modulation modulates light from semiconductor laser by means other than light source. A modulator used in external modulation is generally called an optical modulator. The optical modulator modulates optical intensity, phase, etc. by causing physical changes in the optical modulator based on a signal.
An optical modulator, such as an optical single-side band modulator (optical SSB modulator), and an optical frequency shift keying (optical FSK) modulation method using an optical SSB modulator is described in Tetsuya Kawanishi et al., (Tetsuya Kawanishi and Masami Izutsu, “Optical FSK modulator using an integrated light wave circuit consisting of four optical phase modulator”, CPT 2004 G-2, Tokyo, Japan, 14-16 Jan. 2004) etc.
The environment of an optical modulator may change while the optical modulator is being used. The condition of an optical modulator itself also changes while the optical modulator is being used. If the condition of an optical modulator changes as this way, the optimal operational environment may change as well. Especially, with the above changes, the optimal bias voltage level applied to each MZ waveguide will be changed. Therefore, a system for enabling to adjust the bias voltage level automatically is possibly desired.